A conventional power controller integrated circuit controls a power transistor to couple or decouple a power supply from a load to be powered. The power controller typically detects overcurrent faults, detects voltage levels, detects externally-generated control signals such as an enable signal, and senses other conditions to determine whether to couple or decouple the power supply to or from the load. The overcurrent circuit acts like a circuit breaker, where the current through the transistor is detected and, if it is above a threshold level, the power controller turns off the transistor.
Since the current threshold may be momentarily exceeded due to a current surge when the load is first powered up or due to some other transitory reason, the power controller typically includes a delay circuit which allows the current to exceed the threshold for a short period of time without tripping the circuit breaker. If the overcurrent period exceeds the delay time, the power controller turns off the transistor, and the system must be reset, after the fault is fixed, before the transistor is turned on again. This avoids nuisance faults.
It is important that the delay time during which the overcurrent exists does not exceed a time that will damage the transistor or damage the load or other components. Accordingly, a maximum time for the delay must not be exceeded.
Some power controllers include an autoretry circuit that automatically resets the circuit breaker, after a delay, one or more times after the circuit breaker is tripped in case the previous overcurrent situation has been cleared up. It is important that the reset of the transistor does not occur too soon after the initial shutting off of the transistor since the transistor may heat up and become damaged due to being repeatedly turned on during an overcurrent condition. The ratio of the time that the transistor is on during the overcurrent versus the total cycle time (i.e., the duration of the time that the transistor is on during the overcurrent plus the delay between the autoretry attempt) is referred to as a duty cycle of the transistor during the overcurrent. The duty cycle must not exceed a maximum in order to prevent damage to the transistor.
The delay circuits in the circuit breakers and autoretry circuits may be analog or digital. Analog delay circuits set the delay time by selecting the value of a capacitor that is charged between an initial voltage (e.g., 0 volts) and a threshold voltage. A larger capacitor increases the tripping time. Such capacitors are external to the power controller package and provide flexibility to the user. The analog delay may also be set by selecting an external resistor in addition to the capacitor to set the time constant. Digital timers may be integrated on the same chip as the power controller and are thus inexpensive, but provide no flexibility to the user since the delay time is fixed.
When using an analog timer, the user may err and use a capacitor that provides too much delay and damages the transistor in an overcurrent condition. Similarly, the user may use a capacitor in the analog timer for the autoretry that results in the transistor exceeding its maximum duty cycle for the overcurrent.
What is needed is a technique to prevent the transistor controlled by a power controller being damaged due to a sustained overcurrent or a duty cycle that is too high.